The quest for new oil reserves by the world oil industry forces the industry to seek oil and gas reserves in increasingly more demanding environments including the deep ocean. As the water depth for offshore drilling increases, the size of the equipment required to perform the drilling operations increases, as does the amount of subsea equipment required to extend the well bore to the surface of the ocean. Correspondingly, the costs of the equipment and of the drilling operation increase. A desirable way to offset the increased operating costs resulting from the use of current technology is to increase operating efficiency. An effective way to improve efficiency is to perform operations in less time, which translates into faster operating rates.
Drill pipe is one of the items affected by the increase in water depth. In the early years of offshore drilling, drilling operations were performed in water depths of a few hundred feet using five-inch (12.7 cm.) drill pipe weighing twenty and one-halfpounds per linear foot (30.6 Kg./m.) including the connecting tool joints. Stands of drill pipe made up of three sections of such drill pipe, each nominally thirty-one feet (9.45 m.) in length, are called triples and weigh about one thousand nine hundred (1900) pounds (863.6 Kg). By comparison, triples for deep water drilling operations are made up of five inch (12.7 cm.) drill pipe weighing about 31 pounds per linear foot (46.3 Kg./m.), five and one-half inch (14.0 cm.) drill pipe weighing about 34 pounds per linear foot (50.8 Kg./m.), and six and five-eighths inch (16.8 cm.) drill pipe weighing up to forty-six pounds per foot (68.7 Kg./m.). The weight of drill pipe triples made up of these heavier pipes is about 2880 pounds (1309 Kg.) for the five inch drill pipe, about 3160 pounds (1436 Kg.) for the five and one-half inch drill pipe, and about 4300 pounds (1955 Kg.) for the six and five-eighths inch drill pipe.
The drill pipe used for deep water drilling is made from low alloy steel which has been heat-treated to high strengths. The material is stressed to high levels in use and, therefore, must be maintained free from significant scratches, gouges and other imperfections which can act as stress risers. To get the maximum life out of drill pipe, it must be protected from being scratched and gouged while it is being handled between a pipe storage location and the drill string where it is used. Drill pipe which is damaged beyond rigorous low damage limits must be discarded.
Horizontal pipe rackers commonly are used in floating offshore drilling rigs, especially those of shipform configuration, because they contribute to the floating stability of the rig; they lower the position in the rig of the stored drill pipe. Horizontal pipe rackers store drill pipe triples in a horizontal position and include devices and mechanisms which transport the pipe, in either direction, between the pipe racker and the drill floor. At the drill floor, drill pipe from the racker is moved into a vertical position and is inserted into (connected to) the drill string.
Horizontal pipe rackers currently in use typically store several stands of drill pipe triples in a single bin. When the pipe is put into and removed from the bin, it is rolled down an incline to an indexing device that allows only one stand of pipe to be placed on a transporting device. The rolling produces sliding and impact loading between adjacent drill pipe triples and between drill pipe and the stationary pipe stops. While the pipe is stored in the bins, it can roll back and forth in response to the vessel's own motions thus causing wear and damage between adjacent drill pipe triples. Some pipe rackers prevent the drill pipe movement by storing each stand of pipe in a separate locking mechanism, a tactic that generally limits the drill pipe to only one size. The current method of transporting the drill pipe stands between the drill floor and the drill pipe racker involves sliding the pipe along a long trough causing further wear and scratching along the pipe where it contacts the trough.
U.S. Pat. Nos. 3,083,842 and 3,193,084 pertain to early pipe rackers, versions of which remain in use.
It is seen, therefore, that a need exists for improvements in horizontal drill pipe racking and handling systems to support deep water oil and gas drilling activities. Desirable aspects of such improvements include increased horizontal pipe storage capacity, an ability to accommodate differing diameters of drill pipe in the racker, an ability to store and to handle drill pipe stands in ways which protect the pipe surfaces from being scratched, worn or gouged, and an ability to rapidly and reliably move drill pipe stands between their storage locations and the drill floor.